A conventional ventilation device is provided with a single fan which is rotated in a direction to ventilate a room by a air flow of a predetermined direction. However, if the room is in airtight condition, ventilation of the room is hindered due to the air resistance caused by such condition. Therefore, in order to effectively ventilate a room using a conventional ventilation device, a window or door of the room must be opened.
However, opening the window or door may cause dust or polluted air or even external noise coming in the room, which is unhealthy and unpleasant for people inside the room. Further, since the conventional ventilation device can only function to ventilate the air, another device or devices have to be used to clean the polluted air (air cleaner) and/or warm the air in the room (air conditioner).